Ⅰ.Introduction

As the use of the electronic device is continuously increased such as smartphone, laptop and desktop computer, etc. with the advancement of informatization, the development of science and technology, and the vitalization of entertainment, LED (Light Emitting Diode) display industry is rapidly growing up¹⁾. In research on LED display, although the research for increasing the efficiency of the equipment is being undertaken actively^2~5), it is still concerned about the harmfulness of the short-wavelength out of the visible light spectrum emitted from LED^6,7).

The visible light refers to the electromagnetic wave with 380~750 nm bandwidth detectable by the eyes, and generally the electromagnetic wave in the bandwidth of 380~550 nm refers to short-wavelength called as blue light since it is recognized as blue-based colors in our eyes⁸⁾. As an important element of nature lights the blue light is very important in our lives, but it gives rise to the chromatic aberration caused by high refractivity, the decline of image quality on the retina caused by being scattered by an air particle or an eye medium due to high dispersity, and damage to the visual cells by inducing the generation of reactive oxygen species that result in the photochemical damage when exposing for an extended period of time^9~10).

As the information that LED light contains blue light more than the fluorescent light, incandescent light and sunlight, etc. is released through the internet and mass media, etc., home and abroad manufacturers are producing the lens for blocking blue light, and many people pay attention to it. There is however a difficulty in securing the product reliability because of insufficient evaluation data on the efficiency of the blue light blocking lens¹¹⁾. Although it is thought that blocking the blue light would be effective to the color and the clarity of visual information, the argument on the usefulness is still controversial due to the lack of the research results.

Therefore, in this study we provide the reference data on the blue light blocking lens through analysis of the optical performance of the blue light blocking lens measuring the luminous transmittance and the blue light transmittance, and analysis the influence of the blue light blocking lens on the color accuracy and the clarity of LED display.

Ⅱ.Methods

1.Materials

Eight kinds of lenses manufactured by a coating method (hereinafter “coated lens”) and twelve kinds of lenses manufactured by a tinting method (hereinafter “tinted lens”) still being circulated on the market, which are manufactured by the sphere design having the refractive index of 1.6 and the back vertex refractive power of 0.00 D, were collected for blocking blue light, and the spectral transmittance was measured using the spectrophotometer (CM-3500 D, Minolta, Japan). Eight kinds of coated lenses and eight kinds of tinted lenses having more than 80% of the luminous transmittance were chosen as research subjects of the study by calculating the luminous transmittance (T) and the blue light transmittance (T_B) from the formula (1) and (2) provided by the International Organization for Standardization (ISO 13666, 2012).

T = ∫ 380 780 t ( λ ) V ( λ ) S ( λ ) d λ ∫ 380 780 V ( λ ) S ( λ ) d λ × 100 % T B = ∫ 380 550 t ( λ ) B ( λ ) S ( λ ) d λ ∫ 380 550 B ( λ ) S ( λ ) d λ × 100 %

Where, t(λ) is the spectral transmittance of the lens, S(λ) is the spectral distribution of CIE standard light source D₆₅, V(λ) is the daylight spectral luminous efficiency, B(λ) is the blue light risk function, and λ(nm) is the wavelength.

2.Experiments

The influence of the blue light blocking lens on the color accuracy and the clarity of LED display were measured by camera image quality evaluation equipment, which allow to evaluate the color accuracy and the clarity of the camera.

1)Measurement environment

In the displays with a LED light source, VDT work environment that has high utilization for an extended period of time was set up for measurement environment (Fig. 1).

(1)Measurement chart

Because a PC display is enable to adjust the level of brightness, the influence on the color accuracy and the clarity was measured at the different brightness. In measurement of the minimum, intermediate and maximum brightness controlled by the PC with a LED light source (Nt530U4C-S5HM, Samsung, Korea) using the luminance meter (CX-100, Minolta, Japan), it showed the luminance of 20 nit, 110 nit and 400 nit, respectively. To measure the changes in the color accuracy and the clarity under same condition, the chart that allows to control the quantity of light (SP-STD, Imatest, USA) was applied as the measurement chart, and the luminance was set up with a luminance meter before the experimental measurement. The results of measuring the intensity of light with different wavelengths in the measurement chart using the spectrophotometer (I1 Display pro, X-rite, USA) are represented in Fig. 2., showing that the color temperature of the measurement chart was 5000 K, and the intensity of the light was maximum at 450 nm.

(2)Measurement equipment

The camera applied in the experiment was EOS 650D of Canon, and the camera lens was EF-s 18-55mm F/3.5-5.6 IS II. Under different environment the focal distance, ISO and shutter speed were controlled under same setting condition using the auto-focusing mode. The image was taken using the electronic pressing machine to prevent the movement of camera and to press the shutter with same speed when taking a shot.

(3)Filter of blue light blocking lens

Before the experiment to install the blue light blocking lens in the camera under same condition, 16 UV filter was prepared. The blue light blocking lens was installed after removing the UV lens (Fig. 3).

(4)Illumination system

The illumination of the measurement environment was set up to 200 lx and 400 lx by referring to the VDT illumination of office environment, which is recommended by KS Illumination Standard (KSA 3011:1998) (Table 1 and 2). The illumination value was set up using the illumination system, which enables to adjust the quantity of light (RCLS, Imatest, USA) and the illuminometer (CL-200, Konica minilta, Japan). In addition, the illumination angle of 30°recommended by ISO was maintained (ISO 12233, 2014) to make the light uniform on the measurement chart.

2)Measurement items

3)Image analysis

After analyzing the image filmed with the general camera lens and the image filmed with the blue light blocking lens through the image analysis software (Master 4, Imatest, USA), the changes in the color accuracy and the clarity by the blue light blocking lens were calculated through the following formulas:

Color accuracy change = ( A 1 A 0 − 1 ) + ( B 1 B 0 − 1 ) + ( 1 − C 1 C 0 ) 3 * 100 ( % ) Clarity change = ( 1 − D 1 D 0 ) + ( 1 − B 1 B 0 ) 2 * 100 ( % )

Where, A is the color difference, B is the chroma difference, C is the color reproduction rate, D is the resolution, E is the sharpness, E₀ is the image filmed with the general camera and E₁ is the image filmed with the blue light blocking lens.

It means that when the color difference and the chroma difference are negative (-) and the color reproduction rate is positive (+), the color accuracy is high, and when the resolution and the sharpness are positive, the clarity is high. Thus, the color reproduction rate, the sharpness and the resolution were obtained by calculating the respective values.

3.Statistics

For statistical analysis, SPSS (Ver 18.0, SPSS Inc., USA) was applied. The luminous transmittance and the blue light transmittance of the coated lens and the tinted lens were compared by the Mann-Whitney U test. The regression equation was drawn through the regression analysis after analyzing correlation of the luminous transmittance, blue light transmittance, color accuracy, and the clarity by Kendall’s Tau-b analysis.

For the comparison of the color accuracy and the clarity of the LED display by the blue light blocking lens at different luminance levels, post-hoc test (Bonferroni Correction) was performed after performing the Kruskal-Wallis test. In all the statistical analysis the significance level of 95% was maintained.

Ⅲ.Results and discussions

1.Optical characteristics of blue light blocking lens

The number of subjects applied in our study was 16. The average, minimum and maximum luminous transmittance were 92.91±4.60%, 82.43% and 98.50%, respectively. The average, minimum and maximum blue light transmittance were 80.68± 5.51%, 69.56% and 87.28%, respectively (Table 3).

2.Comparison of optical characteristics of coated lens and tinted lens

In comparison of the luminous transmittance and blue light transmittance of the coated lens and the tinted lens, the luminous transmittance (95.75±1.99%) of the coated lens was 5.67% higher than tinted lens (90.08±4.79%), and the blue light transmittance (82.41±4.43%) of the coated lens was 3.46% higher than the tinted lens (78.85±6.21%). The luminous transmittance showed a significant difference (p<0.05), but the blue light transmittance showed no significant difference (Table 4).

In the correlation between the luminous transmittance and the blue light transmittance, for the coated lens the higher the luminous transmittance, the higher the blue light transmittance is (r=0.714), and for the tinted lens they did not show a significant correlation (Fig. 7). The reason why the coated lens and the tinted lens showed the difference in the optical characteristics would be because the tinted lens blocked visible lights more than 560nm as well as the blue light according to the color and concentration of dyestuff. In the tinted lens, Gre20, Bro20, Gra10 and Gre10 blocked the long wavelength more than the coated lens (Fig. 8).

3.Correlation analysis of luminous transmittance and blue light transmittance

In the correlation between the luminous transmittance and the blue light transmittance, for the blue light blocking lens the higher the luminance transmittance, the blue light transmittance was high significantly (r=0.433) (Fig. 9).

4.Changes in color accuracy and clarity of LED display with blue light blocking lens

In all environments, the color accuracy represented the maximum increase of 1.14%, the maximum decrease of 3.79% and the mean decrease of 1.13±1.37%. In addition, the clarity showed the maximum increase of 4.65%, the maximum decrease of 12.96%, and the mean decrease of 4.95±4.85% (Table 5).

The positive sign is decrement of color accuracy or clarity. The negative sign is increment of color accuracy or clarity.

5.Correlation analysis of changes in optical performance and color accuracy

In the correlation of the changes in the luminous transmittance, blue light transmittance and the color accuracy, no significant correlation was found (Fig. 10).

6.Correlation analysis of changes in optical performance and clarity

In the correlation of the changes in the luminous transmittance, blue light transmittance and the clarity, no significant correlation was found (Fig. 11).

7.Comparison of changes in color accuracy and clarity by luminance

The blue light blocking lens decreased the color accuracy of LED display by 2.34±1.50% at 20 nit and 1.65±2.14 at 110 nit, and increased it by 0.61±1.89% at 400 nit. In addition, it also decreased the clarity of LED display by 9.31±8.95% at 20 nit and 7.79±9.25% at 110 nit, and increased the clarity by 2.26±7.79% at 400 nit. The influence of the blue light blocking lens on the color accuracy and the clarity of LED display was varied at the different luminance levels of display (p<0.05). It represented that it improved the color accuracy and the clarity at high luminance condition (400 nit) whereas decreased the color accuracy and the clarity in the low luminance condition (20 nit and 110 nit), (Table 6).

Ⅳ.Conclusions

The luminous transmittance and the blue light transmittance of the blue light blocking lens were 92.91±4.60% and 80.68±5.51%, respectively and the higher the luminance transmittance, the higher the blue light transmittance was. In the manufacturing methods, the luminous transmittance and the blue light transmittance of the coated lens were higher than those of the tinted lens, and in the coated lens the higher the luminous transmittance, the higher the blue light transmittance was, but in the tinted lens no significant correlation was found between the luminous transmittance and the blue light transmittance. The changes in the color accuracy and the clarity of LED display by the blue light blocking lens did not show a significant correlation with the luminous transmittance and the blue light transmittance, and showed the difference at the different luminance levels of the display. Whereas the blue light blocking lens decreased the color accuracy and the clarity in the low luminance (20 nit and 110 nit), it improved the color accuracy and the clarity in the high luminance (400 nit).

From all results above, it was verified the optical performance and the characteristics of the blue light blocking lens, and also the difference in the influence on the color and the clarity at the different luminance levels of the display. Therefore, it is deemed that the reference data are useful and practical for the performance of the blue light blocking lens.